Damping Performance Analysis and Control of Hybrid AC/Multi-Terminal DC Power Grids

The voltage-source converter (VSC) based multi-terminal direct-current (MTDC) networks are currently being developed in large-scale power grids for efficient and economical transmission of electrical energy generated from remotely sited renewable energy sources (RESs). As the MTDC network is developed alongside the conventional AC grid, it has transformed the power grid to a hybrid AC/MTDC power network. However, since the MTDC network is controlled via the power electronic converters (PECs), it significantly affects the AC power grid damping and synchronising performance as it decouples the natural dynamics between various electro-mechanical systems in the power grid. This paper proposes a supplementary control scheme to improve the damping performance of the entire hybrid AC/MTDC power grid. The damping torque analysis (DTA) technique is used as the primary technique to develop the supplementary control scheme. The preliminary analysis is carried out using a two-terminal hybrid AC/DC power grid and has suggested a supplementary control loop based on the rotor speed deviation for the DC voltage controller of the VSC to improve the damping performance. With the state-space model, the synchronising and damping torque coefficients are calculated and accordingly, the feedback gain is determined to provide optimal synchronising and damping torque components. Subsequently, the fidelity of the supplementary controller is verified using a four-terminal hybrid AC/MTDC grid. Simulation studies prove that the proposed supplementary controller can improve the hybrid AC/MTDC network damping performance, and it performs very effectively with the master-slave control and the conventional droop control scheme.